1. Field of the Invention
The present invention relates to an optical coupler, optical parts and an optical apparatuses utilizing the optical coupler and also a method of manufacturing the optical coupler.
2. Description of the Prior Art
An explosive increase in communication traffic is also expected in the future due to the advent of an Internet society. And implementation of a network capable of accommodating the increase in communication traffic, which is large-capacity, high-speed and inexpensive as to communication costs is required. A network technology meeting such requirements is an optical fiber communication technology. In order to allow large capacity, high speed and inexpensiveness in the optical fiber communication technology, it is necessary to develop a multiplexing technology of high density. While a time division multiplex technology, a wavelength division multiplex technology and combined use of them are thinkable as a method of multiplexing, a direction to adopt the wavelength division multiplex technology (WDM) is mainstream from a viewpoint of easy extensibility. As a concrete approach, in the backbone system (basic network), a WDM technology of such high density as wavelength spacing of zero-point several nm and several tens of GHz for frequency spacing is developed. In the access system (subscriber network) and CATV network, review of methods are underway, such as a method of utilizing both 1.3-μm wavelength light and 1.55-μm light for two-way communication of descending and ascending links and a method of using the 1.55-μm light only for a descending link of a broad band signal while using the 1.3-μm light for two-way communication. Also, in the basic network, in addition to a point-to-point communication system, there are an optical add drop multiplexing (Optical ADM) system for putting signals in and out by wavelengths on a node on the way, an optical cross-connect (Optical XC) system for recombining lightwave paths and besides, an optical routing system for using wavelength information as address information to determine a destination of optical signals and so on so that implementation of a flexible network is expected.
Thus, the optical parts hold the key to implementation of an advanced optical communication system that utilizes wavelengths as a resource. One of the especially important optical parts is an optical multiplexer-demultiplexer, which multiplexes or demultiplexes light waves of different wavelengths to or from a transmission line optical fiber. Representative multiplexer-demultiplexers for high density wavelength multiplexing implemented by the conventional technology are an arrayed waveguide grating (AWG) and a fiber Bragg grating (FBG).
Moreover, as for the optical access system, PDS that performs two-way optical communication between a station and N (a plurality) subscribers via 1:N optical star couplers is a representative example of a network system. And one of technological challenges of the optical parts is that the star coupler sufficiently functions in a descending distribution system but ascending signals from the subscriber lines can only collect power of 1/N at the station in the ascending multiplexing system, which occupies a major portion of signal transmission loss, and so an optical multiplexer of N:1 capable of optical multiplexing with no loss is anticipated.
In addition, another technology requested to be developed in the optical access system is one that allows, in a high-performance and inexpensive manner, implementation of an optical transmission and reception module for two-way communication to be placed on an optical network unit (ONU) on the subscriber side.
In order to implement the above optical parts such as an optical circulator, a 1:N optical coupler and an optical transmission and reception module, it is necessary to develop a new technology that has a nonreciprocal transmission property and yet is implemented at low cost.
Furthermore, the optical parts that are important in implementing the optical communication system are those utilizing the nonreciprocal transmission property of light. The aforementioned optical circulator is also one of the representative nonreciprocal optical parts. The optical circulator is required not only in the above-mentioned form of utilization but also in the case of configuring an optical ADM system for branching light from transmission lines to nodes (terminal equipment) without loss and inversely inserting light signals from the nodes to the transmission lines.
The above-mentioned conventional multiplexer-demultiplexer devices are a device system that artfully utilizes optical interference on a waveguide optical circuit and is configured in a relatively small size with high wavelength resolving power. However, the devices have common faults, that is, they are sensitive to temperature change, increase in optical insertion loss due to connection between the devices and optical fibers cannot be ignored, and they are expensive.
In addition, to embody the optical parts and apparatuses that require the nonreciprocal transmission property including the optical circulator, optical multiplexer with no loss, and optical transmission and reception module mentioned in the above prior art, it cannot be helped, considering the current technological level, to rely on a method of using Faraday polarization rotation effect of magneto-optic materials. Thus, they must be configured by many discrete elements such as lenses and magneto-optic crystals so that they are too expensive and unstable to be practical.